The broad long-term objective of this project is to provide biological imaging onboard radiation treatment machines - that is, biological imaging as the cancer patient is in treatment position. Current onboard target localization is based mainly on x-ray transmission imaging, which is not sensitive to biology and cannot identify some tumors. In addition to target localization, it has also been proposed that dose be modified based on biological function such as hypoxia. Onboard biological imaging - for these tasks - could be provided by single-photon emission computed tomography (SPECT). SPECT is evolving rapidly in its ability to image cancer-relevant biology, such as hypoxia, angiogenesis, cell proliferation, metabolism, epidermal growth factor receptor, apoptosis, and multi-drug resistance. Low-cost SPECT can be engineered to the radiation treatment room. SPECT tracers are widely available, dual-tracer SPECT imaging is possible, and SPECT systems have potential to image positron emitters such as F-18. The onboard SPECT task is to localize biological function within a small volume whose approximate location is known by other localization procedures. Realization of onboard SPECT is challenged by limited time (d 5 minutes) and by geometrical constraints of the treatment room. Novel SPECT methods will be developed to address these tasks and challenges. The Aims are: (1) Develop a prototype system - comprised of a compact nuclear-medicine detector and a robotic arm - that can maneuver the detector about a patient in position for radiation therapy. (2) Design trajectories for onboard parallel-hole and pinhole SPECT imaging of limited volumes, and use the robot/detector system to evaluate task performance of these trajectories. These Aims involve technological developments important not only for onboard SPECT but for SPECT generally. Aim (1) enables detector trajectories that are not possible with conventional SPECT gantries. Aim (2) develops and explores new detector trajectories and the ability of the new hardware to implement them. This work will advance SPECT in novel directions, allowing real-time biological targeting in radiation therapy, and potentially improving treatment outcome.